Investigation into the effects of altitude on phenotypic distribution of Cepaea Nemoralis.
By: Farha Ahmad, Susan Al-Safadi, Clara Holmes, Abbas Omaar, Arash Sayedi and Asma Shirazi Queen Mary University of London
Introduction
Polymorphism is the exhibition of distinct phenotypes within a single population. Cepaea nemoralis (common name Grove snail) is a highly polymorphic species which is a member of the Cepaea genus that falls under the Mollusca phylum. This species has distinct banding patterns and colouring exhibited on the gastropod shells which is a critical feature that allows for it to be easily identified. As a consequence of this and several other strategically advantageous features, C. nemoralis has been a focal point of many studies into the processes involved in evolutionary genetics. Jones et al (1977) commented, regarding the abundance of research concerning C. nemoralis, “we have data on over a million specimens from ten thousand populations, representing a body of information on populations in their natural habitats greater than that which exists for any other animal except man”. This data has only been reinforced and developed further since 1977 and so C. nemoralis continues to be a species critical to the research of mechanisms pertaining to evolutionary genetics. An essential reason why people have been studying C. nemoralis is that the effects of genetic drift, selection, and habitat preferences as well as other evolutionary processes makes it notoriously hard to distinguish which process has affected a difference in phenotypic frequency.
The motility of C. nemoralis is such that travel over even small distances requires relatively long periods of time. In fact, it is estimated that the species moves approximately 30m in space per generation. Exploitation of this has been greatly advantageous to researchers given that the gene frequencies of this species have a higher level of variation at closer distances and, accordingly, sampling can often be confined to a smaller area than normally feasible with other mobile organisms, making research more efficient. Furthermore, the major predator of C. nemoralis, the Song Thrush’s (Turdus philomelos) predation style is to crush the shell of the snail on a surrounding hard surface, such as rock, meaning that very few shells are taken away from the local habitat occupied by the snail. Therefore, sampling C. nemoralis can provide a more accurate representation of the population in situ, as opposed to sampling other species which would not allow for this, as locality to the natural habitat can sometimes not be accounted for as robustly. Other predators of the C. nemoralis species include the glow worm and various rodents.
The aforementioned gastropod shell pattern and colouring varies between the incidence of one of five differing patterns and one of three differing colours. A secondary banding pattern is additionally either exhibited on the shell or not, and the presence of absence of this pattern indicates if an individual C. nemoralis is either in the adult or infant stage of life. A white lip indicates a sub adult, whilst a darker lip would illustrate the snail is an adult. The five banding patterns are quite simply the presence of one band, two bands, three bands, four bands or five bands and the three differing colours are brown, pink or yellow. The secondary banding pattern is comprised of a single band and when exhibited, is in a region of the shell which is distinct from the region where the other banding pattern is shown. In particular we were interested in the colour, banding, and frequency distribution at different altitudes. We chose two different sample areas at different altitudes because we felt this could highlight a difference in phenotypic distribution.
Arnold (1968) concluded that a greater proportion of C. nemoralis exhibited the un-banded phenotype in areas of high altitude in contrast to C. nemoralis at low altitude regions that exhibited the un-banded phenotype to a lesser degree. At transitional altitudes, however, Arnolds noted that the sample populations consisted mostly of the five-banded phenotype and with regards to the colour of the snails it appeared that the frequency distribution of yellow was similar to the un-banded phenotype, with the pink phenotype being found only at intermediate altitudes. Such results were consistent over a large distance and therefore selection was said to be acting upon the C. nemoralis populations in both high and low altitudes. It has been suggested that lower temperatures found at higher altitudes favour the yellow un-banded phenotype. Furthermore, when contrasting areas of open habitats with woodlands within the same study, there was no variation despite the obvious increased risk of predation and so it seems that the colour and banding were two factors affected by area and not selection.
The principle aim of this investigation was to determine the effect of speciation and genetic drift on C. nemoralis. Geneticists have defined genetic drift as being the chance successes of alleles in subsequent generations, i.e. the chance survival of alleles, which has the ultimate effect of altering allele frequencies in successive generations from the expected. Speciation has been defined as the establishment of a species which is incapable of producing fully fertile offspring as a consequence of having mated with another species. An additional aim of this investigation was to establish whether a high altitude or low altitude area has an impacting effect on gastropod shell colour and banding pattern. We anticipate, based on previous findings, that a noticeable difference in frequency of the phenotypes will be seen at high and low altitudes.
The practical dimension of the investigation involved recording differing colour and banding patterns exhibited on shells of C. nemoralis snails at a site in the Buckinghamshire region of England. The methodology entailed the use of a quadrat in order to take several samples in high grass vegetation at low and high altitudes. This choice of methodology was used in order to yield reliable results that would allow for the fulfilment of the aforementioned aims. Tall grass vegetation was found to be more practically suitable than other less suitable vegetation types initially considered such as that of tall trees and large shrubbery. To add, tall grass was found to be the one vegetation that C. nemoralis was most abundant in.
References
Jones, J. S., Leith, B. H., Rawlings, P. (1977). Polymorphism in Cepaea: a problem with too many solutions? Annual Review of Ecology and Systematics, 8, 109-143.
Arnold, R.W., (1968). Studies on Cepaea VIII. Climatic selection in Cepaea nemorclis in the Pyrenees. Philosophical Transactions of the Royal Society of London B, 253, 549-593.
Discussion
The raw data illustrates a general trend of the frequencies of the number of bands increasing at higher altitudes. It is notable that both altitudes featured high frequencies of a higher number of bands (3 - 5 bands). At the high altitude there were a relatively equal number of shells with a high number of banding as well as low numbers of bands. Whereas at the low altitude the majority of shells were highly banded with significantly smaller amounts of low banding on shells. We suggest this difference in banding frequencies is due to the amount of exposure to sunlight the snails receive. At lower altitudes there is less exposure to the sun and so there are fewer lower banded snails. This is because the more bands the snail has, the darker the shell will be, allowing it to acquire energy from the sun at a higher rate. These findings further support those of Arnold (1968) with regards to a higher frequency of unbanded shells exhibited at high altitudes.
Our calculated value of the test statistic, 40.89, is greater than our tabled value, 24.32. Therefore, it is significant and at a 0.1% significance level we can reject the null hypothesis, as there was evidence that the banding frequencies deviated signifcantly from each other at different altitudes (X² = 40.89, df = 7, P < 0.1%).
There are several limitations in our study that need to be addressed. Primarily, as a result of many other research groups sampling on the same site, we cannot be confident that the snails’ shells were not moved from their original position. This made it very difficult to find snails in certain locations, as well as impairing our ability to differentiate between the effects of drift or selection on distribution frequencies as opposed to human interference. This could be improved in future experiments by restricting research groups to certain locations or allowing sufficient time between sampling of a particular area so that it is no longer a factor. A second limitation which could be improved upon in future studies is that of sampling methods. On site, we found our quadrats to be relatively unsuitable; specifically at low altitudes where the snails were more widely distributed, it was difficult to allocate an area with enough snails in a restricted quadrat size. It is known that there are approximately 1.4 adult C. nemoralis per square metre (Hammoud, 2011) and so it is clear to see that larger quadrats are needed to acquire sufficient samples to carry out statistical analysis. In future, we would make our quadrats bigger and with different, better suited material.
In conclusion, we suggest the difference in banding frequencies between altitudes is as a result of selection. Nevertheless, genetic drift is undoubtedly acting on these populations, although these conclustions are not definitive and further studies must be carried out to determine the underlying causes of banding and shell colour polymorphism.
Word count 485 References Arnold, R.W., (1968). Studies on Cepaea VIII. Climatic selection in Cepaea nemoralis in the Pyrenees. Philosophical Transactions of the Royal Society of London B, 253, 549-593. Hammoud, S. 2011. "Cepaea nemoralis" (On-line), Animal Diversity Web. Accessed December 02, 2011 at http://animaldiversity.ummz.umich.edu/site/accounts/ information/Cepaea_nemoralis.html.
Investigation into the effects of altitude on phenotypic distribution of Cepaea Nemoralis.
By: Farha Ahmad, Susan Al-Safadi, Clara Holmes, Abbas Omaar, Arash Sayedi and Asma Shirazi
Queen Mary University of London
Introduction
Polymorphism is the exhibition of distinct phenotypes within a single population. Cepaea nemoralis (common name Grove snail) is a highly polymorphic species which is a member of the Cepaea genus that falls under the Mollusca phylum. This species has distinct banding patterns and colouring exhibited on the gastropod shells which is a critical feature that allows for it to be easily identified. As a consequence of this and several other strategically advantageous features, C. nemoralis has been a focal point of many studies into the processes involved in evolutionary genetics. Jones et al (1977) commented, regarding the abundance of research concerning C. nemoralis, “we have data on over a million specimens from ten thousand populations, representing a body of information on populations in their natural habitats greater than that which exists for any other animal except man”. This data has only been reinforced and developed further since 1977 and so C. nemoralis continues to be a species critical to the research of mechanisms pertaining to evolutionary genetics. An essential reason why people have been studying C. nemoralis is that the effects of genetic drift, selection, and habitat preferences as well as other evolutionary processes makes it notoriously hard to distinguish which process has affected a difference in phenotypic frequency.
The motility of C. nemoralis is such that travel over even small distances requires relatively long periods of time. In fact, it is estimated that the species moves approximately 30m in space per generation. Exploitation of this has been greatly advantageous to researchers given that the gene frequencies of this species have a higher level of variation at closer distances and, accordingly, sampling can often be confined to a smaller area than normally feasible with other mobile organisms, making research more efficient. Furthermore, the major predator of C. nemoralis, the Song Thrush’s (Turdus philomelos) predation style is to crush the shell of the snail on a surrounding hard surface, such as rock, meaning that very few shells are taken away from the local habitat occupied by the snail. Therefore, sampling C. nemoralis can provide a more accurate representation of the population in situ, as opposed to sampling other species which would not allow for this, as locality to the natural habitat can sometimes not be accounted for as robustly. Other predators of the C. nemoralis species include the glow worm and various rodents.
The aforementioned gastropod shell pattern and colouring varies between the incidence of one of five differing patterns and one of three differing colours. A secondary banding pattern is additionally either exhibited on the shell or not, and the presence of absence of this pattern indicates if an individual C. nemoralis is either in the adult or infant stage of life. A white lip indicates a sub adult, whilst a darker lip would illustrate the snail is an adult. The five banding patterns are quite simply the presence of one band, two bands, three bands, four bands or five bands and the three differing colours are brown, pink or yellow. The secondary banding pattern is comprised of a single band and when exhibited, is in a region of the shell which is distinct from the region where the other banding pattern is shown. In particular we were interested in the colour, banding, and frequency distribution at different altitudes. We chose two different sample areas at different altitudes because we felt this could highlight a difference in phenotypic distribution.
Arnold (1968) concluded that a greater proportion of C. nemoralis exhibited the un-banded phenotype in areas of high altitude in contrast to C. nemoralis at low altitude regions that exhibited the un-banded phenotype to a lesser degree. At transitional altitudes, however, Arnolds noted that the sample populations consisted mostly of the five-banded phenotype and with regards to the colour of the snails it appeared that the frequency distribution of yellow was similar to the un-banded phenotype, with the pink phenotype being found only at intermediate altitudes. Such results were consistent over a large distance and therefore selection was said to be acting upon the C. nemoralis populations in both high and low altitudes. It has been suggested that lower temperatures found at higher altitudes favour the yellow un-banded phenotype. Furthermore, when contrasting areas of open habitats with woodlands within the same study, there was no variation despite the obvious increased risk of predation and so it seems that the colour and banding were two factors affected by area and not selection.
The principle aim of this investigation was to determine the effect of speciation and genetic drift on C. nemoralis. Geneticists have defined genetic drift as being the chance successes of alleles in subsequent generations, i.e. the chance survival of alleles, which has the ultimate effect of altering allele frequencies in successive generations from the expected. Speciation has been defined as the establishment of a species which is incapable of producing fully fertile offspring as a consequence of having mated with another species. An additional aim of this investigation was to establish whether a high altitude or low altitude area has an impacting effect on gastropod shell colour and banding pattern. We anticipate, based on previous findings, that a noticeable difference in frequency of the phenotypes will be seen at high and low altitudes.
The practical dimension of the investigation involved recording differing colour and banding patterns exhibited on shells of C. nemoralis snails at a site in the Buckinghamshire region of England. The methodology entailed the use of a quadrat in order to take several samples in high grass vegetation at low and high altitudes. This choice of methodology was used in order to yield reliable results that would allow for the fulfilment of the aforementioned aims. Tall grass vegetation was found to be more practically suitable than other less suitable vegetation types initially considered such as that of tall trees and large shrubbery. To add, tall grass was found to be the one vegetation that C. nemoralis was most abundant in.
References
Jones, J. S., Leith, B. H., Rawlings, P. (1977). Polymorphism in Cepaea: a problem with too many solutions? Annual Review of Ecology and Systematics, 8, 109-143.
Arnold, R.W., (1968). Studies on Cepaea VIII. Climatic selection in Cepaea nemorclis in the Pyrenees. Philosophical Transactions of the Royal Society of London B, 253, 549-593.
Discussion
The raw data illustrates a general trend of the frequencies of the number of bands increasing at higher altitudes. It is notable that both altitudes featured high frequencies of a higher number of bands (3 - 5 bands). At the high altitude there were a relatively equal number of shells with a high number of banding as well as low numbers of bands. Whereas at the low altitude the majority of shells were highly banded with significantly smaller amounts of low banding on shells. We suggest this difference in banding frequencies is due to the amount of exposure to sunlight the snails receive. At lower altitudes there is less exposure to the sun and so there are fewer lower banded snails. This is because the more bands the snail has, the darker the shell will be, allowing it to acquire energy from the sun at a higher rate. These findings further support those of Arnold (1968) with regards to a higher frequency of unbanded shells exhibited at high altitudes.
Our calculated value of the test statistic, 40.89, is greater than our tabled value, 24.32. Therefore, it is significant and at a 0.1% significance level we can reject the null hypothesis, as there was evidence that the banding frequencies deviated signifcantly from each other at different altitudes (X² = 40.89, df = 7, P < 0.1%).
There are several limitations in our study that need to be addressed. Primarily, as a result of many other research groups sampling on the same site, we cannot be confident that the snails’ shells were not moved from their original position. This made it very difficult to find snails in certain locations, as well as impairing our ability to differentiate between the effects of drift or selection on distribution frequencies as opposed to human interference. This could be improved in future experiments by restricting research groups to certain locations or allowing sufficient time between sampling of a particular area so that it is no longer a factor. A second limitation which could be improved upon in future studies is that of sampling methods. On site, we found our quadrats to be relatively unsuitable; specifically at low altitudes where the snails were more widely distributed, it was difficult to allocate an area with enough snails in a restricted quadrat size. It is known that there are approximately 1.4 adult C. nemoralis per square metre (Hammoud, 2011) and so it is clear to see that larger quadrats are needed to acquire sufficient samples to carry out statistical analysis. In future, we would make our quadrats bigger and with different, better suited material.
In conclusion, we suggest the difference in banding frequencies between altitudes is as a result of selection. Nevertheless, genetic drift is undoubtedly acting on these populations, although these conclustions are not definitive and further studies must be carried out to determine the underlying causes of banding and shell colour polymorphism.
Word count 485
References
Arnold, R.W., (1968). Studies on Cepaea VIII. Climatic selection in Cepaea nemoralis in the
Pyrenees. Philosophical Transactions of the Royal Society of London B, 253, 549-593.
Hammoud, S. 2011. "Cepaea nemoralis" (On-line), Animal Diversity Web. Accessed
December 02, 2011 at http://animaldiversity.ummz.umich.edu/site/accounts/
information/Cepaea_nemoralis.html.